This invention relates to the preparation of substantially polycrystalline ceramic fibers from preceramic polymeric precursors, and more particularly to the incorporation of boron into fibers formed from polyorganosiloxane resins to produce high temperature stable silicon carbide fibers.
In recent years ceramic materials have been developed for uses requiring good mechanical strength at high temperatures. Silicon carbide is one such ceramic material which possesses desirable high temperature properties. For example, fibers of silicon carbide have been used as a reinforcing material in composite materials such as fiber reinforced metals and fiber reinforced ceramics. However, the hardness and high temperature properties of silicon carbide make it difficult to fabricate and work with so that various methods of fabrication have been developed depending on the desired form of the final article to be produced.
Many different processes have been used in attempts to manufacture silicon carbide fibers. Some have used inorganic silicon carbide powders as the starting material. However, those processes are useful only for the production of relatively large diameter fibers (approximately 70 micrometers and larger). Smaller diameter fibers are more desirable because they are more flexible, can be woven, and provide better reinforcement of metal and ceramic matrix materials.
Where it is desired to produce small diameter fibers of silicon carbide, one method which has been used is to spin an organosilicon polymer into a fiber. The fiber is then infusibilized to render it nonmelting (typically by air treatment at somewhat elevated temperatures) followed by pyrolysis at high temperatures to produce a ceramic fiber.
A problem in the preparation of silicon carbide fibers by the above method is that substantial amounts of oxygen and or nitrogen may either already be present in or introduced into the fibers during spinning, infusibilization, or ceramification. The presence of this oxygen and/or nitrogen adversely affects the thermal stability of the fibers. That is, as the fibers are ceramified at high temperatures, the oxygen and/or nitrogen present in the fibers leaves the fibers, causing weight losses, porosity, and losses in tensile strength in the fibers. While lower ceramification temperatures may be used to decrease the amount of oxygen and/or nitrogen lost, exposure of such fibers to high temperatures during use results in the same problem of the oxygen and/or nitrogen present in the fibers leaving the fibers, causing weight losses, porosity, and losses in tensile strength in the fibers.
Workers have attempted to minimize the oxygen and/or nitrogen present in ceramics fabricated from organosilicon polymers by using the class of such polymers known as polycarbosilanes (PCS). The polymeric backbone structure of polycarbosilanes consists of only silicon and carbon as opposed to polyorganosiloxanes in which the polymeric backbone structure consists of silicon and oxygen. For example, Yajima et al, U.S. Pat. No. 4,100,233, teaches a process for the production of silicon carbide fibers using polycarbosilanes as a starting material.
Nicalon (trademark), a commercially available silicon carbide containing ceramic fiber based on a polycarbosilane starting material, is produced by the above-described process of fiber spinning, infusibilization, and then high temperature pyrolysis. However, the Nicalon fibers so produced contain significant amounts of oxygen (9-15 % by weight). It is known that Nicalon's mechanical properties degrade at elevated temperatures as low as 1200.degree. C. due to weight losses and porosity as the oxygen and/or nitrogen leaves the fibers.
Some workers have incorporated other elements into silicon carbide-based bodies derived from polycarbosilanes in an attempt to improve the mechanical properties of the bodies. Thus, elements such as boron, titanium, and zirconium have been introduced into preceramic polymers. Yajima et al, U.S. Pat. No. 4,248,814, teaches sintering a polycarbosilane and up to 15% by weight of a borosiloxane polymer to produce a ceramic.
Yajima et al, U.S. Pat. No. 4,359,559, teaches the production of a polymetallocarbosilane by mixing a polycarbosilane with a titanium or zirconium containing organometallic compound. Yajima et al, U.S. Pat. No. 4,347,347, teaches the production of a block copolymer of a polycarbosilane and a polymetallosiloxane. Yajima et al, U.S. Pat. No. 4,342,712, teaches the production of titanium, silicon, and carbon-containing ceramic fibers from a block copolymer of a polycarbosilane and a titanoxane. Yajima et al, U.S. Pat. No. 4,152,509, teaches the incorporation of boron into the backbone of a polysiloxane to form a borosiloxane polymer which is then mixed with a powdered silicon carbide and cold pressed into a molded article.
However, the prior art describes problems with the incorporation of these elements (sometimes termed heteroatoms) into the polymer. For example, the synthesis procedures for heteroatom incorporation involve high temperature and pressure reaction conditions. The yields of the resulting polymers are low. Also, the heteroatoms bond to the silicon atoms in the polymer backbone through intermediate oxygen linkages so that increasing amounts of oxygen are present in the polymer. Further, silicon carbide-based fibers so produced are typically composed of extremely fine crystalline grains; heating the fibers to temperatures of 1300.degree. C. or higher causes growth of the grains which results in a decrease in mechanical strength of the fibers. See, Takamizawa et al, U.S. Pat. No. 4,604,367 at column 1.
Takamizawa et al, U.S. Pat. No. 4,604,367, teaches the preparation of an organoborosilicon polymer by mixing an organopolysilane with an organoborazine compound, spinning fibers, and then ceramifying the fibers by heating at temperatures in the range of from "900.degree. to 1800.degree. C.". However, the actual examples in Takamizawa show heating up to only 1500.degree. C., and the tensile strength of the Takamizawa fibers is reported to drop off dramatically when heated to temperatures approaching 1500.degree. C.
Takamizawa et al, U.S. Pat. No. 4,657,991, teaches the formation of ceramic precursors of silicon carbides using a polycarbosilane and an organometallic compound containing boron, aluminum, titanium, or zirconium. The patentee teaches pyrolysis of the polymer at temperatures between about 800.degree. and 1500.degree. C. Pyrolysis temperatures above 1500.degree. C. are taught to decrease the mechanical strength of the resulting fibers due to grain size growth.
However, there are a number of applications for ceramic fiber materials which must be able to withstand exposure to much higher temperatures above 1500.degree. C. while retaining their mechanical strength properties. Thus, there remains a need in the art for thermally stable, small diameter silicon carbide fibers for use in both metal and ceramic matrix composites which can withstand very high temperatures of operation.